1. Field of the Invention
The present invention relates to an improved method of crystallizing 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.05,903,11]-dodecane, also known as hexanitrohexaazaisowurtzitane, hereinafter referred to as CL-20. In particular, this method involves crystallization of CL-20 as an epsilon-polymorph.
2. Description of the Related Art
For most existing propellant and weapons systems, the most critical ingredient in terms of propulsive and explosive performance is the oxidizer. CL-20, with its substantial increase in performance output, is an organic oxidizer presenting significant opportunities in terms of energy capabilities for propellants and explosives. For example, the use of CL-20 as the energetic filler in weapons systems may provide increased anti-armor penetration, enhanced missile payload velocity and flight, increased underwater torpedo effectiveness and lethality, and improved gun propellant impetus.
The performance of CL-20 in propellant and weapon systems is highly dependent upon the crystal polymorph of CL-20. CL-20 has several different crystal polymorphs, the most preferred of which is a high density phase known in the art and referred to herein as the xcex5-polymorph (or epsilon-polymorph) of CL-20. The xcex5-polymorph of CL-20 is preferred because of the high energetic performance and relatively low sensitivity attributable to the xcex5-polymorph. However, many conventional CL-20 synthesis techniques produce xcex1-polymorph as the predominant crystal polymorph. The xcex1-polymorph has a much lower density that the xcex5-polymorph. For these reasons, CL-20 synthesized by many conventional techniques must be subjected to re-crystallization in order to increase the concentration of the xcex5-polymorph.
Conventionally, CL-20 has been crystallized using chloroform to precipitate CL-20 from ethyl acetate. Chloroform has been found to produce consistently and reproducibly the desirable xcex5-polymorph of CL-20. However, one disadvantage to using chloroform is that defects are often found in the crystalline structure of xcex5-polymorph CL-20 crystallized with chloroform. Another disadvantage of this conventional technique is that chloroform and ethyl acetate cannot be separated effectively and efficiently by distillation, thus complicating the reuse of these solvents. Because the chloroform cannot be easily reused, a continual discharge of a chlorinated waste stream must be disposed of in an environmentally acceptable manner. As a chlorinated solvent, chloroform may potentially contribute to ozone depletion, thus complicating waste disposal of chloroform and other chlorinated solvents. It is, therefore, advantageous to crystallize CL-20 into the xcex5-polymorph with solvents that can be recycled within the crystallization process without producing a discharge of chlorinated solvents.
A CL-20 crystallization technique that avoids the use of chloroform and other chlorinated solvents and non-solvents is disclosed in U.S. Pat. No. 5,874,574, in which CL-20 is dissolved in a solution containing a CL-20 solvent, such as ethyl acetate, and water to form an aqueous phase and a wet solvent phase. The wet CL-20 solvent phase is then dried by azeotropicly. A low density CL-20 non-solvent is then added to the dry CL-20 solvent phase to cause crystallization of xcex5-polymorph CL-20. The CL-20 crystals are then separated from the non-solvent and the solvent by adding sufficient water to displace the non-solvent and the solvent from the surface of the xcex5-polymorph CL-20 crystals. Although high recoveries of xcex5-polymorph CL-20 are reported in U.S. Pat. No. 5,874,574, it is also disclosed that relatively large quantities of water are needed to separate the nonsolvent and solvent from the CL-20 crystals. In some cases, the quantities of water can require larger separation and recycling equipment, thus increasing the capital expenditures and operating costs of this process.
It would therefore be a significant improvement in the art to provide a method for crystallizing high concentrations of xcex5-polymorph CL-20 having high quality and little defects without relying on chlorinated solvents and non-solvents or large water separation and recycling equipment needed by conventional processes.
It is, therefore, an object of this invention to overcome a long-felt need in the art by providing a method that produces xcex5-polymorph (epsilon-polymorph) CL-20 possessing excellent quality in high yields, yet which method is environmentally friendly and more economically efficient than known methods.
In accordance with the principles of this invention, the above and other objects are attained by a method in which CL-20 is crystallized from a solution comprising at least one CL-20 organic solvent and a CL-20 non-solvent comprising at least one nitrate ester, in particular poly(glycidyl nitrate) and/or a nitrate ester plasticizer. The nitrate ester is preferably poly(glycidyl nitrate) and/or triethyleneglycol-dinitrate, although other nitrate plasticizers having acceptable volatilities and impact sensitivities can be used. The solution is saturated with CL20, and CL-20 is crystallized from the saturated solution by, for example, adding xcex5-polymorph CL-20 crystalline seeds to the solution and evaporating off the CL-20 solvent. Evaporation is preferably conducted under vacuum or with the aid of a similar technique for removing the solvent vapor, such as blowing a dry gas over the evaporator. The nitrate ester non-solvent and any non-evaporated remnants of the solvent are then separated from the crystalline CL-20 by a suitable solid-liquid separation technique, such as by filtration of the CL-20 crystals. If necessary or desirable, prior to solid-liquid separation the nitrate ester non-solvent can be diluted, and its viscosity lowered, by diluting the slurry of non-solvent and CL-20 with a solvent that is miscible with the non-solvent but in which the CL-20 is insoluble. The CL-20 can then be washed.
Advantageously and unexpectedly, the crystallization of the CL-20 in the nitrate ester non-solvent produces high quality xcex5-polymorph CL-20 crystals that may have few crystal defects and exhibit enhanced energetic performance and lower impact sensitivity compared to CL-20 crystallized by known techniques. Additionally, the solution in which the CL-20 is dissolved and eventually crystallized comprises a mixture of an environmentally acceptable solvent and a non-solvent free of chlorinated compounds and other compounds regulated as Hazardous Air Pollutants (HAPs) under the Clean Air Act. Both the solvent and non-solvent can be recycled for further processing without further treatment or purification.
The CL-20 crystallized by this method is excellent for use in propellant, explosive, and pyrotechnic formulations.
Other objects, aspects, and advantages of this invention will become more apparent to those skilled in the are upon reading the specification and appended claims, which explain the principles of this invention.
Crystallization of CL-20 in accordance with this novel method is performed in a solution comprising at least one CL-20 organic solvent and a CL-20 non-solvent comprising at least one CL-20 nitrate ester that is miscible with the solvent.
As referred to herein, the term xe2x80x9cCL-20 solventxe2x80x9d includes solvents that have a relatively high CL-20 solubility of at least 20% weight/volume (g/ml) of CL-20 in the solvent. The CL-20 solvent preferably has a relatively low boiling point to permit evaporation of the CL-20 solvent at temperatures not exceeding 60xc2x0 C. Solvent evaporation can be, and preferably is, conducted under a vacuum or in the presence of a blowing dry gas or the like to remove the solvent vapor. Ethyl acetate is currently the preferred solvent because of its low boiling point and environmental acceptability compared to chlorinated solvents. Other non-halogenated CL-20 solvents suitable for use in this invention include other alkyl acetates, such as methyl acetate, n-propyl acetate, and iso-propyl acetate; ketones such as acetone, methyl ethyl ketone; cyclic ethers such as tetrohydrofuran; nitromethane; and acetonitrile. Preferably, an effective amount of the organic solvent is included in the solution mixture to completely dissolve the CL-20 ingredient into the solution prior to commencement of crystallization.
Nitrate esters that are suitable for use in the present invention as the CL-20 non-solvent include those nitrate esters having relatively low vapor pressures, low volatilities, and low impact sensitivities for nitrate esters. The selection of nitrate esters meeting these criteria prevents evaporation of the nitrate ester non-solvent as the CL-20 solvent is evaporated, even when the evaporation of the CL-20 solvent is conducted under vacuum or with the aid of a dry gas. Evaporation of the nitrate ester is desirably avoided due to the hazardous associated with evaporation of nitrate esters. The non-solvent selected for use in this invention is preferably poly(glycidyl nitrate) (PGN) and/or the nitrate ester plasticizer triethyleneglycol-dinitrate (TEGDN), which may be used alone or in combination with each other or other non-solvents. Other suitable nitrate ester plasticizers, such as butanetrioltrinitrate (BTTN) and diglycerol tetranitrate (DGTN), can also be used, although these alternative nitrate ester plasticizers are less preferred due to their higher impact sensitivities. Although other nitrate plasticizers may be used, care should be taken due to their high vapor pressures and volatilities, which can cause these nitrate esters to evaporate with the CL-20 solvent and complicate separation of the solvent and non-solvent.
The weight ratio of nitrate ester non-solvent(s) to CL-20 is preferably not less than about 4:1, and more preferably is in a range of from about 5:1 to about 8:1. The presence of less than an about 4:1 ratio of nitrate ester non-solvent to CL-20 can lead to the formation of defects in the CL-20 during crystallization. On the other hand, operating at a ratio of more than about 8:1 is economically inefficient inasmuch as such high ratios may require longer processing times, more man-power, and larger operating equipment.
The nitrate ester non-solvents can be used alone or in combination with other non-solvents known in the art. In the event that the poly(glycidyl nitrate) or nitrate ester plasticizer(s) are used with known non-solvents, the non-solvents preferably have poor CL-20 solubility of not more than 5% weight/volume (g/ml), more preferably not more than 1% weight/volume (g/ml), of CL-20 in the non-solvent. The CL-20 non-solvent preferably possesses low volatility and has a boiling point higher than that of the solvent, thus permitting the solvent to be separated from the CL-20 by evaporation or the like while retaining the non-solvent. The non-solvent(s) should be present in a sufficiently low amount to avoid precipitation of the CL-20 out of the solution mixture prior to the addition of CL-20 crystalline seeds and/or evaporation of the solvent(s).
In selecting a CL-20 solvent and CL-20 non-solvent, consideration should be given to the boiling point differential, which preferably is at least 20xc2x0 C.
Dissolution of the CL-20 into the solvent is preferably conducted at about the same temperature at which the CL-20 is crystallized. Any sequence can be selected for combining the CL-20, the solvent, and the nitrate ester non-solvent, although preferably the CL-20 is added either to the solvent alone or to a mixture of the solvent and nitrate ester non-solvent. The solution is saturated with CL-20, for example, by adding a sufficient volume of non-solvent to the solution to reach the saturation point. Other suitable techniques for saturating the solution include, by way of example, the introduction of additional CL-20 to the solution and/or evaporation of a portion of the solvent from the solution.
As referred to herein, saturated solution encompasses solutions at their saturation points or exceeding their saturation points (i.e., supersaturated), so long as the solution is not supersaturated to the extent that the solution self-nucleates prior to the addition of CL-20 crystal seeds. Excess super-saturation and self-nucleation can cause crystal growth to occur at an extremely high rate, leading to a large number of crystal defects.
In a preferred embodiment of this invention, xcex5-polymorph CL-20 is crystallized out of the saturated solution by adding xcex5-polymorph CL-20 seed crystals to the solution and evaporating off the solvent from the solution. The CL-20 crystal seeds are preferably not more than about 30 xcexcm in diameter, more preferably about 2 xcexcm in diameter. To obtain CL-20 crystal seeds in this range, CL-20 crystals can be ground or milled by techniques known in the art, such as a fluid energy mill or a ball mill. The quantity of CL-20 crystal seeds to be added to the saturated solution depends upon the desired crystal sizes of the crystals to be grown. An example of an effective CL-20 crystal size is about 150 xcexcm diameter crystals.
Evaporation of the solvent is preferably done in a azeotropic manner to simultaneously remove any water present in the solution. The azeotropic evaporation of the solvent can be performed subsequent to introducing the CL-20 crystal seeds into crystallization medium or, in the alternative, the sequence of these steps can partially or completely overlap with one another. However, the CL-20 seeds are preferably added to the crystallization medium before the occurrence of excessive super-saturation, which may be caused by evaporation of large amounts of the solvent without simultaneous crystallization of CL-20 out of solution.
High crystallization rates are also avoided by selection of an acceptable crystallization temperature, preferably in a range of from about 25xc2x0 C. to about 60xc2x0 C. Operating at temperatures higher than 60xc2x0 C. can lead to undesirably high rates of crystallization and can lead to the formation of polymorphs other than the epsilon polymorph. On the other hand, evaporating the solvent at temperatures lower than about 25xc2x0 C. can cause unacceptable amounts of solvent to remain with the CL-20 crystals. Failure to remove a substantial portion of the CL-20 solvent (e.g., about 90 wt % of the CL-20 solvent) can also result in defects in the crystallinity of the CL-20. Selection of the optimum crystallization temperature will vary depending upon the boiling point of the solvent. Another way to facilitate the formation of CL-20 crystals having low defect is to stir the solution at a relatively slow rate during evaporation.
In an alternative, less preferred embodiment of this invention, it is possible to crystallize the CL-20 out of solution without the use of CL-20 crystal seeds. However, the degree of super-saturation of the solution should be maintained low, such as by evaporating off a small portion of the CL-20 solvent. If held for a sufficient period of time, the supersaturated solution will self-nucleate, prompting crystal growth without the addition of CL-20 crystal seeds.
Subsequent to removal of the solvent and crystallization of the CL-20 in the nitrate ester non-solvent, the crystalline CL-20 is separated from the nonsolvent. Separation can be conducted by known solid-liquid separation techniques. For example, the CL-20 crystals can be filtered from the non-solvent, then washed. In the event that a viscous nitrate ester non-solvent is selected, such as poly(glycidyl nitrate), it may be necessary or desirable to lower the viscosity of the non-solvent by diluting the solution in another CL-20 non-solvent. Representative organic liquids for washing the CL-20 crystals and, where appropriate, lowering the viscosity of the nitrate esters, include alcohols such as isopropanol and ethanol, and ethers such as dialkyl ethers, especially diethyl ether. Chlorinated solvents, such as methylene chloride, can also be used, although the chlorinated solvents are less preferred because of the environmental problems raised by their use.
The crystallization method of this invention can be performed on CL-20 made by any one of various techniques known in the art. In this regard, the disclosures of CL-20 synthesis set forth in U.S. Pat. No. 5,693,794 to Nielson, U.S. Pat. No. 5,739,325 to Wardle et al., and EP 0 753 519 A1 are incorporated herein by reference. Another technique for preparing CL-20 comprises nitrolysis of 2,6,8,12-tetraacetyl-2,4,6,8,10,12-hexaazatetracyclo-[5.5.0.05,903,11]-dodecane (xe2x80x9cTADHxe2x80x9d) in a mixed acid comprising nitric and sulfuric acids at 85xc2x0 C. The volumetric ratio of nitric acid to sulfuric acid is preferably about 7:3. The ratio of mixed acid (in milliliters) to TADH (in grams) is preferably about 8:1.
Prior to conducting the crystallization method of this invention, the CL-20 feed can be pre-treated to neutralize any residual acids, such as nitric and sulfuric acids. A representative neutralizing agent is sodium bicarbonate.
The crystallized CL-20 can then be combined with appropriate amounts of binder, plasticizer, fuel, inorganic oxidizers, curative, and/or other ingredients known in the art to make a propellant or explosive. The preparation of propellants and explosives, including the selection of appropriate ingredients and processing steps, is known in the art. Generally, CL-20 constitutes up to about 50% by weight of the total weight of a cured propellant, more preferably not more than about 25% by weight for propellants. Higher concentrations of CL-20 are often desirable for explosives and the like.